In order to design an FMS(Flexible Manufacturing System), an effective design tool is necessary for selecting and anlayzing the complex interacting factors of a manufacturing system. Considering most of the previous researches have been focused on evaluating the designed FMS, development of an integrated FMS designing system is needed for performing the design task such as machine selection and layout determination as well as the evaluation task of the designed FMS. In this paper, we developed a prototype of such a system called the intelligent FMS designer based on the techniques of Expert System and Simulation. By using this designing system, possible design alternatives can be analyzed and appropriate manufacturing capacity can be estimated at the designing stage. Various operating characteristics of the designed FMS can be also predicted before it is implemented. Eventually a suitable design alternative can be provided with cost details within relatively short time period. To demonstrate the performance and effectiveness of the designer, a simple example is tested. The results show a great potential for automating design processes of an FMS.